This invention relates to microwave signal routing matrices of the type having N inputs and M outputs and known as crosspoint or crossbar switches.
Crosspoint or crossbar switches have been used for many years in telephone applications in order to connect an arbitrary one of a plurality of inputs to an arbitrary one of a plurality of outputs. Such crosspoint switches are readily implemented at telephone frequencies, because the switching takes place within a region which is a small fraction of a wavelength in extent, and because the effect of reactances associated with the switching elements is small at telephone frequencies. It may be desirable to accomplish crosspoint switching at microwave frequencies, as for example to route signals around failed elements, for reconfiguring microwave circuits, or for reconfiguring the elements of an array antenna in order to select appropriate radiation characteristics.
Microwave signals are ordinarily carried by conductors configured in the form of a transmission line. The salient feature of a transmission line lies in the maintenance of a substantially constant distributed inductance and capacitance between the conductors along the length of the conductors. This is ordinarily accomplished by maintaining a fixed cross-sectional shape along the length of the conductors. FIG. 1a illustrates a transmission line of the coaxial (coax) type. This well known type of transmission line includes a hollow cylindrical outer conductor 12 centered on a longitudinal axis, together with a relatively thin center conductor 14 extending along the axis. As illustrated in FIG. 1a, center conductor 14 is connected to outer conductor 12 by a short-circuiting conductor 16 at the right-hand end, and is open at the left-hand end. As is well known in the art, the length in wavelengths along a transmission line equals the free-space wavelength at the frequency in question multiplied by a fraction representing the relative velocity of propagation along the transmission line, which is generally proportional to the square root of the effective dielectric constant. The illustrated length of the coaxial structure is an odd multiple of one-fourth wavelength, or more formally EQU L=(2N+1).lambda./4 (1)
As is well known to those skilled in the art, the apparent impedance looking into the left side of transmission line 10 is an open circuit (infinite ohms) at a frequency for which the above condition exists. As the frequency departs from that at which the condition is exactly met, the impedance at the left end transmission line 10 decreases. When such circuits are used, their operation is normally specified to include a range of frequencies around the frequency for which Equation 1 holds.
FIG. 1b illustrates another type of transmission line known as microstrip. Microstrip transmission line 20 of FIG. 1b includes a flat dielectric plate 22 on the bottom side of which is fastened or deposited a conductive ground plane 24. A strip conductor 26 extends along the upper side of dielectric plate 22. As in the arrangement of FIG. 1a, one end of strip conductor 26 is connected by way of a conductive through path or via 28 to ground plane 24. At a point along strip conductor 26 of microstrip transmission line 20 which is at a distance (2N+1) .lambda./4 from via 28, the impedance is at a maximum. FIG. 1c is a symbolic representation of a coaxial transmission line specifically and transmission line in general.
U.S. Pat. No. 3,833,866 issued Sept. 3, 1974 to Boutelant describes a microwave switching matrix of the crosspoint type in which each crosspoint connection is made by a diode coupled between transmission lines. Biasing of the diode is accomplished by means of inductors configured as quarter-wavelength transmission lines. The Boutelant arrangement uses an isolator associated with each input port and with each output port to reduce the effect of standing waves attributable to reflections at the crosspoints. In one embodiment, Boutelant uses power dividers as isolators. Such isolators or power dividers introduce cost and loss penalties.
The arrangement of FIG. 2a illustrates a well known arrangement which provides the function of a single pole, double throw switch 200 for microwave signals. In the arrangement of FIG. 2a, a microwave source illustrated as an oscillator 210 is coupled by way of an input port 212 and a common input transmission 214 to a junction point 216. Junction point 216 is connected to a junction point 222 by way of a switchable output transmission line including portions 218a and 218b separated by a DC blocking capacitor 220. Junction point 222 is coupled by a further blocking capacitor 224 to an output port 226. While the terms input port and output port suggest a preferred direction for the flow of signal, those skilled in the art realize that the direction of signal flow is irrelevant, and the direction of signal flow and designation of the ports could as well be reversed.
A direct-current-passing, high-frequency rejecting filter illustrated as an inductor 228 is coupled at one end to junction point 222 in FIG. 2a. A switching diode 230 is connected between junction point 222 and ground. In this context, ground represents the ground plane or the outer conductor of the appropria&e transmission line. Junction point 216 is coupled to a junction point 242 by a transmission line including portions 238a and 238b separated by a blocking capacitor 240. Junction 242 is coupled by way of a blocking capacitor 244 to a switched output port 226. A low-pass, high-reject filter represented as an inductor 248 is coupled to junction point 242. A diode 250 is connected between junction point 242 and ground. Inductors 228 and 248 are connected to a switching control circuit illustrated as a block 250 which produces appropriate bias signals for diodes 230 and 250 for switching control. Junction points 222 and 242 are each one-fourth wavelength from junction point 216. It is expected that input and output ports are connected to sources and loads having impedances which are substantially matched to the characteristic impedances of the transmission lines over the frequency range of interest.
In operation, switching control circuit 250 applies current by way of either inductor 228 or 248 for forward-biasing one of diodes 230 or 250. When forward biased, the diode assumes a low impedance condition and effectively short-circuits between the conductors of the associated transmission line. As illustrated in FIG. 2a, the physical length of the diode symbol for a diode such as 230 is a significant proportion of the length of the transmission line represented by segments 218a and 218b. However, in actual practice, the physical length of the diode is very small by comparison with the quarter wavelength. In operation with one of diodes 230 or 250 forward-biased, the effective short circuit produced by the forward-biased diode appears as an open circuit or high-impedance condition at common junction point 216. The diode which is not forward biased remains open-circuited. It should be noted that the open circuiting of a diode such as diode 250 does not result in open-circuiting of the transmission line.
The transmission line to which the open-circuited diode is connected has its impedance established by the termination coupled to its output port. Signal from generator 210 flows to that output port which is associated with an open-circuited diode. That is, if diode 230 is forward biased and therefore a short circuit, and diode 250 is open circuited, signal flows from input port 212 to output port 2. Similarly, if diode 250 is forward biased and therefore short-circuited, and diode 230 is not biased and is therefore an open circuit, signal flows from common port 212 to output port 1. The arrangement of switch 200 has the advantages of simplicity and low through loss.
It should be noted that in the description of microwave circuits, switching elements such as diodes 230 and 250 may be represented as mechanical switches, such as illustrated in FIG. 2b. In FIG. 2b, switching diode 250 of FIG. 2a is symbolically represented by a corresponding mechanical switch symbol. When the symbolic representation is used, the biasing networks and blocking capacitors are not ordinarily shown.
The arrangement of FIG. 2a has a single input and two outputs. It would be very desirable to be able to connect an arbitrarily large number of input ports to an arbitrarily large number of output ports using a simple low-loss arrangement such as that of FIG. 2a. FIG. 2c is a redrawing of the arrangement of FIG. 2a to facilitate comparison of its topography with that of the invention.